The present invention relates, in general, to micron-scale optoelectronic devices, structures and techniques, and more particularly to devices and structures for facilitating the interaction of optical components such as optical fibers with other fibers and/or with circuit components such as wave guides or active elements such as light sources or light detectors on or connected to micromechanical structures.
Recent developments in micromechanics have successfully led to the fabrication of devices in single crystal substrates utilizing a dry etch process such as reactive ion etching (RIE) for producing micron-scale moveable mechanical structures. Such a process is described in U.S. Pat. No. 5,198,390 as utilizing multiple masks to define small, complex structural elements and related elements such as metal contacts in single-crystal silicon. U.S. Pat. No. 5,393,375 describes a similar process for releasing micromechanical structures in single-crystal materials other than silicon. An improved dry-etch process for the fabrication of microelectromechanical structures is described in U.S. Pat. No. 5,846,849, which discloses a single-mask, low temperature, self-aligned process wherein discrete devices can be made, and wherein such devices can be fabricated in wafers containing integrated circuits. The processes described in these patents may be used to produce a variety of sensor devices such as accelerometers, as well as a variety of actuator devices, resonators, moveable optical reflectors, and the like, either as separate, discrete devices or as components on previously-fabricated integrated circuits. The processes described in these patents may be referred to in general as the SCREAM (Single Crystal Reactive Etch and Metal) process, with the single mask process being referred to as the SCREAM-1 process.
As the field of micromechanical and microelectromechanical devices developed, a problem arose concerning the connection of ultra small components and structures formed on a wafer or substrate with other circuits and components on other wafers or substrates, whether of micron-scale or larger. One solution has been to fabricate solder pads on these devices for use in securing connecting lines or wires to the electrical components on the substrate. However, such a procedure requires precision wire bonding techniques which do not always produce satisfactory results. Furthermore, the use of wires for communication with microcircuits and related devices limits the flow of data between the circuits and devices.
On the other hand, optical fibers provide many advantages in data communication, but problems are encountered in connecting small diameter optical fibers to micromechanical devices such as waveguides and light detectors for transferring data to circuits carried by the substrate, as well as for transferring data from such circuits, as by way of laser light sources on the substrate. A major problem is that of alignment of fibers with each other, with microstructures such as waveguides and reflectors, with light sources such as vertical cavity surface emitting laser (VCSEL) arrays, and with electrical circuit components such as light detectors or the like.
The alignment of VCSEL arrays and detector arrays for direct coupling to optical fiber arrays is challenging, because the fibers must be mounted with their axes perpendicular to the light emitter or detector. The fiber support structure thus must be perpendicular to the detector or emitter, and the fabrication of micromechanical supports for this purpose is difficult.
Misalignment between fibers or between a fiber and a device or structure can occur in three translational directions and can occur around three rotational axes. Optical interconnections are most sensitive to lateral misalignment; that is, misalignment in directions perpendicular to the direction of propagation of light in the fiber, but the connections are also sensitive, to a lesser degree, to angular misalignment and to the axial distance between components in the direction of propagation. For single-mode optical systems such as those employed in telecommunications applications, lateral misalignment between optical components should be less than one micrometer, while for multimode systems, lateral misalignment tolerances are more relaxed; for example, up to about 5 micrometers. In both cases, axial separation tolerances are often greater by a factor 2-5, depending on the components involved. Single-mode interconnections typically can tolerate small angular misalignments; for example, less than 0.5 degree, depending on coupling efficiency requirements. In the case where columnated beams of light are coupled, where the beam waist is often 10-100 times the diameter of typical single-mode fiber beam profiles, angular misalignment of matching beams must be much smaller; for example, less than 0.01 degree. In all cases an accurate alignment is essential to effective, reliable communication.
Accordingly, there is a need for structures and devices for accurately, reliably and easily interconnecting optical fibers with each other, with micromechanical devices and structures and with light detectors and emitters.
Briefly, the present invention is directed to improved methods and apparatus for easily and accurately interconnecting small-diameter optical fibers in end-to-end axial alignment. The invention is further directed to micron-scale fabrication techniques and to passive optical components fabricated by such techniques for connecting such optical fibers to micromechanical and to microelectromechanical devices such as waveguides and for optically coupling such fibers to electrical circuits by way of active optical elements such as light detectors and laser sources.
The packaging of optical fibers with micromechanical and microelectromechanical devices is carried out, in a first embodiment, by mechanical couplers for connecting optical fibers in end-to-end alignment so as to obtain a maximum transfer of laser light energy or the data carried by such light energy from one optical fiber to another. Such couplers may be used to interconnect a single pair of fibers, or may be used to connect an array of optical fiber pairs, with the couplers providing easy and accurate assembly.
In another embodiment of the invention, an optical coupler interconnects one or more optical fibers with mechanical or electrical components carried by a substrate. The electrical components may be active elements such as light sources or light sensors, for example, which to are electrically connected to corresponding circuit components such as integrated circuits carried by the substrate. Such a coupler may incorporate trenches for receiving and holding optical fibers in alignment with suitable waveguides or reflectors for directing light carried by the optical fibers to corresponding detectors or sensors. In another alternative, the circuits or components on the substrate may consist of light sources such as a solid state lasers which generate light in response to signals from electrical circuits on the substrate, with the light produced by the lasers being directed into the optical fibers by way of the waveguides or reflectors.
In a preferred form of the invention, alignment of optical fibers with active optical components such as optoelectric detectors or laser light sources is attained by securing the optical fiber or fibers in a first substrate, which will be referred to herein as a coupler block. A second substrate, which will be referred to herein as a substrate or a wafer, and which contains the light detectors or light sources, is secured to the coupler block. The substrate may be mounted on or above, and parallel to, the surface of the coupler block, with its active optical components (light detectors or light sources) positioned in alignment with corresponding fibers. Alternatively, the wafer may be edge-mounted on or in the coupler block, as in a trench formed in the coupler block, or may be mounted on an edge of the coupler block. In order to ensure alignment of the detectors and light sources with the optical fibers, the trench for receiving the substrate must be precisely shaped and accurately located, and the substrate must be held firmly in place. In accordance with the invention, various mounting devices, including fasteners, springs, and the like, are provided to align the substrate with the optical fibers in the coupler block and to secure it in place.
In the preferred form of the invention, the various mounting devices consist of micromechanical structures fabricated in the connector block, which preferably is a single to crystal silicon substrate. The mounting structures are unitary with the connector block, and are fabricated by one of the SCREAM micromachining processes described above so that all of the trenches, connectors, fasteners, springs, waveguides, reflectors, and like structures which make up the connector block of the invention are fabricated in a single process.
The SCREAM-1 process utilizes a single crystal substrate of a material such as silicon, gallium arsenide, silicon germanium, indium phosphide, compound and complex structures such as aluminum-gallium, arsenide-gallium-arsenide, and other quantum well or multi-layer super lattice semiconductor materials in which moveable, released structural elements electrically isolated from surrounding substrate materials and metallized for selective electrical connections can be fabricated using a single mask. The structures fabricated by the SCREAM processes can be discrete; i.e., can be fabricated in a substrate or wafer form ed from any of the aforementioned substrate materials. The processes allow structures to be fabricated in silicon wafers containing integrated circuits, since the SCREAM processes use a low temperature dry etch procedure.
Complex shapes can be fabricated by the SCREAM processes, as illustrated in the ""849 patent, including triangular and rectangular structures, as well as curved structures such as circles, ellipses and parabolas for use in the fabrication of fixed and variable inductors, transformers, capacitors, switches and the like. Released, cantilevered structures can be fabricated by this process for motion along x and y axes in the plane of the substrate, along a z axis perpendicular to the plane of the substrate, and for torsional motion out of the plane of the substrate.
The SCREAM processes in a single crystal substrate permit formation of deep, narrow trenches which may be located and oriented as desired, and which can be used to define isolated and released structures and to produce high aspect ratio structures. In addition, the processes permit deep lateral etching extending below any structures which are to be released, and can be used to produce extended cavities in the sidewalls of mesas adjacent trenches or surrounding released structures. The released structures can include single or multiple fingers cantilevered to side walls of the substrate and extending outwardly over a trench bottom wall, as well as various grids and arrays, and various electrical components. The various structures may be referred to herein as xe2x80x9cbeamsxe2x80x9d or as xe2x80x9creleased beamsxe2x80x9d.
In accordance with the SCREAM-1 process, a dielectric mask layer of oxide or nitride is deposited on the top surface of a wafer or substrate, using a standard PECVD process. Preferably, the substrate is single crystal silicon, with the dielectric layer serving as a mask throughout the remainder of the steps. The standard PECVD process is used because of its high deposition rate and low deposition temperature. Thereafter, a resist layer is spun onto the mask layer, and standard photolithographic resist techniques are used to produce in the resist layer a pattern which defines the desired micromechanical structure. The pattern in the resist is then transferred to the mask dielectric layer using, for example, CHF3 magnetron ion etching (MIE) or RIE. An O2 plasma etch may be used to strip the resist layer, and a deep vertical reactive ion etch (RIE) or a chemically assisted ion beam etch (CIAB) is used to transfer the pattern from the dielectric mask into the underlying wafer to form trenches which define, in top plan view, the outline of the desired structures, with the trenches being from 4 to 20 micrometers deep and having substantially smooth, vertical walls.
After completion of the trenches, a protective conformal layer of PECVD oxide or nitride is applied to cover the silicon structures to a thickness of about 0.3 micrometers, for example. The conformal dielectric layer covers the top surfaces of the substrate as well as the sides and bottom walls of the trenches. Thereafter, the conformed dielectric layer is removed from the trench bottom wall, as by an anisotropic RIE which removes the previously applied 0.3 micrometers of dielectric from the substrate top surfaces and from the trench bottom, leaving the trench side wall coatings undisturbed. As a result, the substrate is left with a top surface and side wall layer of dielectric, with the bottoms of the trenches being free of dielectric.
A deep RIE or CAIBE is used to etch the floor of each trench down below the lower edge of the side wall dielectric to thereby expose the substrate material below the dielectric on each side of the trench. An isotropic RIE is then used to etch the substrate material laterally under the dielectric layer on the side walls to form cavities. If the trenches define beams or other narrow structures, the lateral etching may extend completely under the beams or narrow structures to release them, while cavities will be formed under other fixed (nonreleased) structures, which may be referred to as mesas. The etch chemistry has high selectivity to the dielectric, allowing several microns of substrate to be etched without appreciably affecting the protective dielectric coating. Released beams are thus cantilevered over the bottom wall of the deep silicon trench, with the cantilevered structures having a core of semiconductor material and a conformal coating of dielectric on their top surfaces and side walls. If desired, a metal layer may be deposited onto the structure, as described in U.S. Pat. No. 5,846,849.
The SCREAM-1 process permits fabrication of high aspect ratio microstructures with precise geometries, is compatible with existing semiconductor fabrication techniques, and is preferred, although other bulk micromachining processes can be used.
In its simplest form, the optical coupler block of the invention connects a pair of optical fibers in an end to end relationship. The coupler block is fabricated by a micromachining process such as the SCREAM-1 process to etch a trench, or fiber guide, across a silicon substrate or wafer to define the location of the two fibers. The fiber guide is flared where it meets opposite edges of the block to create tapered receptacles which receive the ends of the optical fibers to be aligned and direct them into the fiber guide. The guide dimensions are selected to firmly receive the optical fiber so that when one fiber is inserted into the guide from each end, the fibers will be aligned at the center of the guide where they abut. If desired, a precision stop can be etched into the guide to control the distance that each fiber travels when being inserted, and a multiplicity of such guides may be formed in the coupler block to allow alignment of multiple pairs of fibers. The coupler block may be fabricated from a stand-alone substrate, or may be fabricated by micromachining it in a larger substrate; for example, to form the coupler block in a cavity in the substrate surface.
In another embodiment of the invention, instead of aligning fiber pairs, the optical coupler block is modified to couple optical fibers to passive optical components such as waveguides, reflectors, or the like, or to active electrooptical components such as light sources or light detectors. In this case, a fiber guide is formed so that it extends from a flared receptacle at the edge of a substrate into a corresponding cavity having a vertical wall where the guide terminates. In one form of the invention, the cavity is fabricated to incorporate a sloped reflective wall aligned with the optical fiber guide so that light entering the cavity from an optical fiber in the guide will be reflected upwardly towards an opening on the surface of the substrate. The coupler block may support a separate, surface-mounted substrate or wafer carrying an optically active element such as light detector which may be aligned with the upwardly opening reflector in the cavity so that the optical fiber is in communication with the active element. The separate wafer may contain, or may be connected to, external circuitry or may be connected to circuitry on the coupler block itself. Alternatively, or in addition, the optically active element on the separate wafer may be a surface emitting laser which emits light into the cavity when the wafer is mounted on the coupler substrate, with the laser light then being reflected toward the corresponding optical fiber guide.
In other embodiments, the wafer may be mounted on an end of the coupler block, with one or more optical fibers extending across the coupler for alignment with corresponding detectors or surface emitting lasers on the end-mounted wafer. Further, instead of incorporating a reflector, the cavity in the coupler may comprise a waveguide for coupling light from an optical fiber to detectors or to other optical components on a substrate, or on the coupler block itself.
In a preferred form of the invention, optical fibers are coupled to active optical elements on a substrate by edge-mounting the substrate in the coupler block so that the axes of the fibers are perpendicular to the substrate surface on which the active elements are mounted. This ensures that the light from the fibers will strike the surfaces of the corresponding active elements at right angles, or the light from such elements will be parallel to the axes of the corresponding fibers, for maximum efficiency. Careful, precise alignment of a substrate carrying optical elements in or on the coupler block is critical to assuring reliable optical coupling between the optical elements and an optical fiber, and accordingly a variety of alignment techniques have been devised, in accordance with the invention. Exemplary techniques and structures for ensuring accurate alignment of edge-mounted substrates include precision etching (i.e, within plus or minus 1 or 2 micrometers) of a deep cavity or trench having the dimensions required to accurately position an edge-mounted wafer in the coupler block. The wafer is positioned in the trench with its surface perpendicular to the fibers and with the active elements aligned with corresponding fibers. Once positioned it may be bonded in place, but it has been found that thermal expansion can cause undue stresses in the microstructures, resulting in deformations which adversely affect optical coupling. Preferably, therefore, the wafer is aligned and secured within the trench by microsprings fabricated when the trench is formed. The springs may be provided with tabs or rings to permit retraction for release of the wafer, but operate to firmly hold the wafer in a selected position for alignment while accommodating changes in dimensions due to temperature variations. The present invention contemplates a wide variety of alignment springs, including edge springs, corner springs and keyed springs.
Alignment can be further assured by the provision of notches, pits or depressions formed on the wafer for receiving and locating the alignment springs, and such notches may be tapered or nontapered to receive corresponding pins or tips fabricated in the connector substrate. If desired, alignment, grooves or trenches can be located on the wafer to guide the alignment springs into corresponding notches and the tips of the alignment springs may be tapered, flared or burred to hold them in place. Vertical alignment of the wafer may be provided by suitable stops or shoulders formed in the etched trench to engage corresponding notches on the wafer. These techniques can be used to align one or more wafers in the coupler substrate, as required.
In order to connect the electrical components carried by the wafer to external circuits, various wire bonding techniques may be utilized, or conventional solder ball interconnections may be used. Thermal stress relief may be provided by mounting the connections on flexible spring-beams, if desired, or the connection can be provided by means of a metallized spring tip engaging a contact pad on the wafer.
The foregoing fabrication and mounting techniques provide a compact electrooptical connector package in which optical fibers are accurately and reliably aligned with other fibers or electrooptical components carried by a wafer, and to structures wherein optical components are electrically connectable to corresponding circuits carried by the coupler substrate or other wafers. Angular and lateral alignment of optical fibers is carried out during the lithographic patterning steps, in accordance with the invention, so that various components are effectively self-aligned to a high degree of accuracy. Further, alignment structures such as tips, notches, fiber guides, and the like, are designed to compensate for variations in etching by providing symmetrical designs. As a result, when both sides of an alignment structure etch at the same rate, the remaining portion is automatically aligned with a lithographically-determined reference.